You can treat a date as a continuous quantity after placing the field on a shelf. You do this by selecting one of the Continuous date options on the field’s context menu (lower list of date levels). Continuous dates draw a quantitative axis for the date values.
For example, the view below displays the sales as a function of a continuous order date and is color-encoded by department. As you can see, the color of the Order Date field changes from blue to green after it is converted to a continuous quantity.
Treating dates as a continuous quantity is particularly useful when you use Gantt bars or want to see trends using line charts as shown above.
By default, date dimensions are discrete fields for which Tableau automatically selects a date level when it is placed on a shelf. To make a date dimension continuous by default, right-click the field in the Data window and select Convert to Continuous. The field turns green and is automatically converted to a continuous field when you drag it to a shelf. To revert to discrete again, right-click the field in the Data window and select Convert to Discrete.